Process of coating metals with bismuth or bismuth-base alloys



United States Patent iiFice 2,821,595 Patented Jan. 28, 1958 PROCESS OF COATING METALS WITH BISMUTH R BISMUTH-BASE ALLOYS John G. Beach, Columbus, Ohio, assignorto the United States of America as represented by the United States Atomic Energy Commission No Drawing. Application April 1, 1954 Serial-No. 425,338.

7 Claims. (Cl. 2041-37) This invention deals with a process of coating metal articles with bismuth or bismuth-base metals, both being referred to hereinafter as bismuth metal.

In order to start out neutronic reactors the use of a neutron source is desirable. For power-producing reactors this source frequently comprises a beryllium target ghzigh is bombarded with alpha particles derived from P0 is produced by neutron irradiation of bismuth whereby Bi is formed (Bi (n, 'y)Bi which then decays with emission of ,8" particles to P0 In order to materialize these reactions, a metal base, preferably aluminum, for instance, is coated with bismuth or a bismuth-base metal, and the unitobtained thereby is exposed to a neutron-irradiation whereby P0 is obtained.

Such a bismuth-metal-coated metal unit has to fulfill various requirements to warrant satisfactory operation in the neutronic reactor. In the first place, the bismuth has to be well bonded to the metal base; secondly, the layer has to be dense and uniform and also should not be brittle. Another desirable feature is a line grained structure. Moreover, the bismuth layer should be stable at the elevated temperatures to which it will be exposed in the reactor. Since the melting point of bismuth is relatively low, namely, at 217 C., it has been found desirable for some purposes to alloy the bismuth with another metal which will raise the melting point. On the other hand, however, the quantity of the alloying metal should not exceed about 10% by weight, so that, apart from the melting point, the properties of the bismuth are not remarkably changed. A great many alloying ingredients have been studied, but have been found unsuitable for one reason or another. Nickel, however, was found to yield an alloy of the desired properties, and a content of about 9% by weight yields an alloy with a solidus point of 469 C.

The production of a nickel-bismuth alloy was found to cause various difiiculties. For instance, in order to obtain homogeneity, the production temperature had to be raised to about 900 C., a temperature at which the bismuth starts to evaporate to a considerable amount. Coating of the metal base, in particular of aluminum, the preferred metal for this purpose, with the nickelbismuth alloy, too, was difiicult, and various problems had to be solved to arrive at a satisfactory process. For instance, it had been tried to cast the bismuth-nickel alloy on an aluminum sheet at a temperature between 550 and 600 C. in an inert and dry hydrogen atmosphere and at a reduced, pressure of 3 microns of mercury. However, the aluminum was attacked extensively by the bismuth alloy, and wettability as well as adhesion were found unsatisfactory. According to another process tested, an. intermediate nickel coating was applied, prior to coating with the alloy, at about 600 C. under similar conditions as were used in the process of coating with the alloy; this was believed to improve adhesion.

However, in order to obtain a satisfactory metal coating in this case, the nickel-bismuth alloy had to be heated about the peritectic which, in contact with the intermediate nickel layer, caused the formation of a liquid phase too low in nickel. Moreover, the temperature required for coating with the alloy was too close to the melting point of aluminum (657 C.).

In order to coat a metal base with pure bismuth, electro-deposition, using direct electric current, has been investigated. The deposits obtained, however, had a coarse-grained and dendritic structure. Formation of Bio" and Bi(OH) and precipitation of bismuth hydroxide cause inclusions in the metal deposit and these, in turn, spongy, poorly adhering coatings.

In order to overcome this drawback, it had been suggested to add organic compounds such as glue and/or sulfonated cresol to the electrolyte. By these additions the deposits were improved; they were fine-grained and bright. However, they were too brittle, contained organic occlusions and during heating showed a tendency to form blisters and shatter.

It is an object of this invention to provide a process for producing coatings of bismuth metal on a metal base whereby a deposit of uniform thickness is obtained.

It is another object of this invention to provide a process for producing coatings of bismuth metal on a metal base whichrhave a fine-grained, structure.

It is another object of this invention to provide a process for producing coatings of bismuth metal on a, metal base in which the addition of organic. compounds is unnecessary.

It is another object of this invention to provide a process for producing coatings of bismuth metal on a metal base which are free of occlusions and which in particular do not contain any bismuth hydroxide.

It is another object of this invention to provide a process for producing coatings of bismuth metal on a metal base which are not brittle.

It is still another object of this invention to provide a process for producing coatings of bismuth metal on a. metal base which adhere firmly to the base. metal.

It is finally also an object of this invention to provide coatings of bismuth metal on a metal base which are stable at temperatures up to about 450 C.

These and other objects are accomplished by' immersing the metal base to be, coated in an aqueouselectrolytic bath containing bismuth chloride, sodium chloride and hydrochloric acid, passing an electric current through said bath while alternately making the base the cathode and the anode, the duration of the cathodic phase of each cycle exceeding that of the anodic phase;

Since bismuth compounds have a great. tendency to precipitate in the form of bismuth hydroxidev in dilute aqueous solutions, no matter whether they are acid or alkaline, it was found preferable first to dissolve the bismuth chloride and sodium chloride in concentrated hydrochloric acid and then only to add the water necessary to obtain the desired degree of dilution. By following this procedure, the precipitation of bismuth hydroxide is prevented.

The concentrations of the ingrcdicntsof the bath may vary widely; however, the preferred electrolyte contained, per liter of electrolyte, grams of bismuth chloride, 18.5 grams of sodium chloride and2l0 m1. of a.3.8,%. hydrochloric acid;

Room temperature was suitable for the. process of this invention, and temperatures between 21 and 32 C. were found best. The current density may vary, but the preferred conditions were 30 amps/sq. ft. While the process is operative as long as the cathodic phase is.longer than the anodic, it'was preferred to have the cathodic phase of the cycle last about twice as long as the anodic phase. Under these conditions a deposit of a thickness of 1 mil was deposited in 33 minutes. The cathode efliciency was 100% and the over-all efiiciency 65%. By repeatedly reversing the current in short intervals, a uniform deposit was obtained.

The metal base is preferably surface-cleaned prior to depositing the bismuth metal. Any method known to those skilled in the art is suitable for this purpose; however, cleaning by electropolishing was preferred.

The process of this invention lends itself also to the production of a nickel-containing bismuth alloy. For this purpose first a layer of nickel and then a layer of bismuth were applied to the metal base by electrolysis, and the unit was then subjected to difiusion heating whereby alloy formation between the metals of the two layers took place.

Prior to electrodeposition of the nickel layer on an aluminum base, the aluminum was preferably given a cleaning pretreatment, for instance with an organic solvent, and/or with an organic detergent or an alkaline solution, and/ or by electropolishing, for instance, by the process of U. S. P. 2,550,544 granted to Charles L. Faust on April 24, 1951. Anodic pickling with aqueous hydrochloric acid followed by chemical pickling with sulfuric acid was also found suitable, either alone or in c'ombina tion with any other pretreating step or steps.

The nickel was deposited by making the aluminum the cathode in a nickel sulfate-containing electrolyte. The deposit was surface-activated by electropolishing and/or by cathodic treatment in hydrochloric acid and thus made ready for the bismuth layer. The electropolishing of the nickel layer was preferably carried out according to the process disclosed and claimed in U. S. P. 2,440,715 granted to Charles L. Faust and Paul D. Miller on May 4, 1948.

In depositing the bismuth layer on the nickel coating, it was found advantageous to start with the anodic phase of the cycle; replacement deposition of the bismuth on the nickel was thereby prevented. The bismuth deposition was effected by the procedure described above in connection with the application of bismuth to aluminum. It was found advisable, though not necessary, in order to obtain an especially smooth and thick deposit, to rinse and brush the deposit from time to time during deposition.

While one nickel layer and one bismuth layer are sufficient for the purpose of this invention, it was found preferable to deposit a plurality of thin alternate layers and then form the alloy by difiusion heating. Prior to depositing nickel onto the bismuth layer previously formed, a film of bismuth hydroxide, which had formed during rinsing, is advantageously removed to obtain better adhesion. This is preferably done by immersion in hydrochloric acid followed by rinsing with cold water. Thereafter the nickel is deposited as described above.

After all the layers have been applied to the base metal, the entire unit is subjected to diffusion heating. In the beginning this was carried out at a temperature as high as 175 C., but the results obtained thereby were not satisfactory; the layers separated at several points upon cooling. After compresentive experimentation it was found that the best results were obtained when heating was carried out at up to about 150 C., preferably at 90 C. and under reduced pressure, for instance of about microns of mercury, for approximately 70 hours.

It will be understood that, although the invention has been specifically described as applied to the coating of an aluminum carrier, the base may be of a great variety of other metals, depending upon the purpose and use desired. For instance, a base of nickel has also been successfully used. 'The material of the base is not part of the invention.

The bismuth-coated units produced by the process of this invention are bombarded with neutrons, e. g., in a neutronic reactor, and then assembled with a berylliumcontaining body so that the alpha particles emanating 4 from the Po formed in the reactor impinge on the beryllium and thereby cause the liberation of neutrons.

In the following an example is given of the process as applied to the production of a nickel-bismuth alloy on an aluminum base. This example is merely for illustrative purposes, and it is not intended to have the invention limited to, or by, the details given therein.

Example A rectangular aluminum plate, 5% inches by 10 inches, was degreased with an organic solvent and then scrubbed with a detergent solution. Thereafter it was rinsed with cold water, subjected to electropolishing according to the per liter of solution, 145 grams of NiSo .7H O; 75 grams I of MgSO .7I-I O; 15 grams of NH Cl; 15 grams of H BO and 20 ml. of sodium lauryl sulfate. The temperature of the bath was 32 C. and the current density used was 15 amps/sq. ft. Under these conditions the deposition rate was 1 mil in minutes. Electrolysis was carried out for 4 hours. The nickel deposit, which was 0.003 inch thick, was rinsed with cold water and electropolished for surface activation and then rinsed once more (U. S. P. 2,550,544).

Further surface activation was then effected by cathodic treatment for 30 seconds in a 3.8% hydrochloric acid at room temperature using 10 amps./ sq. ft. The article was then ready for superimposing the bismuth layer.

For the bismuth deposition an electrolyte was used which contained 100 grams of bismuth chloride, 18.5 grams of sodium chloride and 210 ml. of a 30% hydrochloric acid, all per liter of solution. The temperature of the bath during electrolysis was 26 C., and the nickelcoated aluminum was first made the anode. Anodic andcathodic treatment were alternated and the times used were 5-and 10.5 seconds, respectively. Electrolysis was carried out for 132 minutes, which means until a deposit of a' thickness of 0.004 inch had been obtained. The unit was then again immersed in a 3.8% hydrochloric acid and treated cathodically under the same conditions aswere applied to the cathodic pickling step for the nickel coating. The member was then rinsed in cold water, dried and heated at C.'at 5 microns of mercury for 70 hours whereby the uppermost nickel layer and the bismuth deposit formed an alloy which, upon examination, showed a uniform, fine-grained, and homogeneous structure and was well bonded to the aluminum.

The process of this invention is suitable for the coating of articles of various shapes and configuration; however, it proved particularly advantageous for the coating of articles or members which have curved surfaces or contours because no other process has been found applicable heretofore for the satisfactory plating of such articles with bismuth or bismuthnickel alloys.

It will be understood that this invention is not to be limited to the details given herein but that it may be modified within the scope of the appended claims.

What is claimed is: 1. A process of producing a layer of a binary nickelbismuth alloy on a metal base comprising immersing the' metal base in a nickel sulfate-containing bath, electrog lyzing the bath while making the metal basethe cathode" until a nickel coating of the desired thickness has been obtained, thereafter immersing the nickel-coated base in a solution of an electrolyte mixture consisting essentially of grams of bismuth chloride, 18.5 grams of sodium chloride and 210 milliliters of a 38% hydrochloric acid per liter of solution, eleetrolyzing the solution while male ing the nickel-coated base the anode, reversing the current and alternating between cathodic and anodic treatment of the base until a bismuth coating of the desired thickness has been deposited, and alloying the nickel coating and the bismuth coating by diflusion heating.

2. The process of claim 1 wherein the metal base is aluminum.

3. The process of claim 1 wherein diifusion heating is carried out at from about 90 C. to 150 C. at subatmospheric pressure.

4. The process of claim 3 wherein the temperature is about 90 C.

5. A process of producing a layer of binary nickelbismuth alloy on a metal base comprising immersing the metal base in a nickel sulfate-containing bath, electrolyzing the bath while making the metal base the cathode until a nickel coating of the desired thickness has been obtained, thereafter immersing the nickel-coated base in a solution of an electrolyte mixture consisting essentially of 100 grams of bismuth chloride, 18.5 grams of sodium chloride and 210 milliliters of a 38% hydrochloric acid per liter of solution, electrolyzing the solution while making the nickel-coated base the anode, reversing the current and alternating between cathodic and anodic treatment of the base until a bismuth coating of the desired thickness has been deposited, alternately depositing a plurality of nickel and bismuth coatings, and alloying the nickel and bismuth coatings by diffusion heating.

6. The process of claim 5 wherein the article is immersed in hydrochloric acid after each bismuth coating.

7. A process of producing a coating of a nickel-containing bismuth alloy on an aluminum base comprising surface cleaning said aluminum base; subjecting the base to anodic treatment for from 2 to 5 minutes in a 3.8% hydrochloric acid at from 21 to 32 C., using a current density of about 50 amps/sq. ft.; immersing the pickled base in a dilute sulfuric acid at room temperature for from 2 to 5 minutes; rinsing the base with cold water; cathodically treating the base in an electrolyte containing,

per liter of solution, 145 grams of NiSO .7H O, grams of MgSO .7H O, 15 grams of NH C1, 15 grams of H 30 20 ml. of sodium lauryl sulfate, using a current density of about 15 amps/sq. ft. and a temperature of about 32 C.; rinsing the nickel-plated base with cold water; electropolishing the coated base; again rinsing with cold water; cathodically pickling the coated base in a 3.8% hydrochloric acid of room temperature for 30 seconds, using a current density of about 10 amps./sq. ft.; immersing the nickel-coated article in a solution containing, per liter, 100 grams of BiCl 18.5 grams of NaCl and 210 ml. of a 38% hydrochloric acid while repeatedly alternating between anodic and cathodic connection of the coated base, each cathodic phase lasting for about 10 seconds and each anodic phase for about 5 seconds; rinsing the bismuth coating obtained with cold water; immersing the article obtained in a 3.8% hydrochloric acid; rinsing with cold water; repeating these various steps for alternately applying nickel and bismuth layers until the desired number of layers has been obtained; and heating the article at about C. at a pressure of about 5 microns of mercury whereby a homogeneous alloy is formed between the various layers.

References Cited in the file of this patent UNITED STATES PATENTS 130,362 Lobstein Aug. 13, 1872 2,441,810 Glower May 18, 1948 2,451,340 Jarnstedt Oct. 12, 1948 2,515,192 Chester July 18, 1950 2,569,149 Brennan Sept. 25, 1951 2,637,686 McKay May 5, 1953 2,682,702 Fink July 6, 1954 OTHER REFERENCES Modern Electroplating, edited by Allen G, Gray, John Wiley & Sons, New York 1953, pages 511, 512, 517, 518, 488, 489, 320.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2321 505 January 28, 1958 John Go Beach It is hereby certified that error appears in the printed specification of. the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1 line 40, for "217 0.," read 271 0 column 2, line 3, for "about" read above column 3, line 60 for compre'sentive read mm comprehensive 0 Signed and sealed this 26th day of May 1959o (SEAL) Attest:

KARL H, AXLINE ROBERT c. WATSON Attesting Oificer Commissioner of Patents 

1. A PROCESS OF PRODUCING A LAYER OF A BINARY NICKELBISMUTH ALLOY ON A METAL BASE COMPRISING IMMERISING THE METAL BASE IN A NICKEL SULFATE-CONTAINING BATH, ELECTROLYZING THE BATH WHILE MAKING THE METAL BASE THE CATHODE UNTIL A NICKEL COATING OF THE DESIRED THICKNESS HAS BEEN OBTAINED, THEREAFTER IMMERISING THE NICKEL-COATED BASE IN A SOLUTION OF AN ELECTROLYTE MIXTURE CONSISTING ESSENTIALLY OF 100 GRAMS OF BISMUTH CHLORIDE, 18.5 GRAMS OF SODIUM CHLORIDE AND 210 MILLILITERS OF A 38% HYDROCHLORIC ACID PER LITER OF SOLUTION, ELECTROLYZING THE SOLUTION WHILE MAKING THE NICKEL-COATED BASE THE ANODE, REVERSING THE CURRENT AND ALTERNATING BETWEEN CATHODIC AND ANODIC TREATMENT OF THE BASE UNTIL A BISMUTH COATING OF THE DESIRED THICKNESS HAS BEEN DEPOSITED, AND ALLOYING THE NICKEL COATING AND THE BISMUTH COATING BY DIFFUSION HEATING. 